An off-grid generator set is a power generating system whose connection to an electrical power grid is temporarily or permanently unavailable. Off-grid generator sets have been widely used. For example, a portable/standby generator set may be utilized to power electrical equipment connected via a circuit with the generator set when people do camping, tailgating and working outside. A similar power generating apparatus is also utilized to provide backup power in emergencies, for example during blackout.
The generator set usually utilizes a driving engine coupled to an alternator (or generator) through a common shaft. Upon actuation of the engine, the engine rotates the common shaft to drive the alternator that, in turn, generates electrical power. Most residential electric equipment is designed to be used in connection with electrical power having a fixed frequency, for example, 60 hertz (Hz) in North America and 50 Hz in China. The frequency of the output electrical power is determined by the operating speed of the engine. As a result, the engine's operating speed of some generator sets is fixed as so to keep the output electrical power at a fixed frequency. However, when an electrical load applied to the generator set is less than the rated kilowatt load for which the generator set is designed, the engine's fuel-efficiency deteriorates and the generator set generates loud noise.
It is therefore desirable to have a generator set or power generating apparatus that allows a variable engine speed, an output voltage with a constant amplitude and a constant frequency.